This application is based on Patent Application Nos. 124345/1998 filed on May 7, 1998 in Japan, and 132825/1998 filed on May 15, 1998, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical fiber, more specifically to a transmission medium used in optical communication networks and optical signal processing.
2. Description of the Prior Art/Related Art
FIG. 1 is a sectional diagram showing the construction of a prior art optical fiber. In FIG. 1, numeral 11 denotes a core, 12 is a cladding, and 13 is a jacket.
FIG. 2 shows a refractive index profile of the prior art optical fiber shown in FIG. 1. Numerals 2axe2x80x2, 2bxe2x80x2, and 2c represent diameters of the core, cladding, and jacket, respectively, of which typical values of practical fibers are 4 xcexcm, 15.9 xcexcm, and 125 xcexcm, respectively.
xcex94n2 and xcex94n3 represent a refractive index difference between the core 11 and the jacket 13 and a refractive index difference between the cladding 12 and the jacket 13, respectively, of which typical values of both are 0.75% and 0.11%, respectively.
Prior art optical communication fibers are composed mainly of quartz glass both for the core and cladding, and a dopant material such as GeO2 or P2O5 is added to the quarts glass of the core to increase the refractive index of the core, so that the optical power is concentrated on the core part for propagating the light in the optical fiber.
In the prior art optical fiber, since the refractive index of the core 11 of the fiber is higher than that of the cladding 12, light incident to the optical fiber is confined in the core 11 of the fiber due to the refractive index difference and propagates in the optical fiber. For achieving the confinement of light by the refractive index difference, to satisfy the single mode condition of propagating light, the core diameter is as small as about 4 m. However, in association with the advance of the optical communication networks and optical signal processing, it is required to provide a high capacity optical fiber.
T. A. Birks, et al, in xe2x80x9cEndlessly single-mode photonic crystal fiberxe2x80x9d, Optics Letters, vol. 22, No. 13, pp. 961-963, 1997 and xe2x80x9cSingle-mode photonic crystal fiber with an indefinitely large corexe2x80x9d, Technical Digest of the 1998 Conference on Lasers and Electro-optics, CWE4, pp. 226-227, disclose an optical fiber made of quartz glass having a core part without hollow hole and a cladding part having hexagonal arranged hollow holes. According to the literature, although the core diameter is larger than that of the prior art optical fiber, single-mode characteristics can be maintained. Also in this optical fiber, the refractive index of the cladding is smaller than that of the core, and light is therefore confined by total internal reflection as in the prior art optical fiber having no hollow hole.
When short optical pulses or high-power optical signals are propagated in such a prior art optical fiber, there are various disadvantages due to the core made of quartz glass. That is, due to absorption and scattering by impurities in the quartz glass, and nonlinear optical effects of quartz glass when the peak power of the optical signal confined in the core exceeds about 10 mW, spectrum width of the optical signal is increased by a self-phase modulation effect, and incident power is limited by Brillouin Scattering. As a result, deformation of optical waveform and saturation of incident power to the optical fiber occurs. Accordingly, transmission characteristics of optical signals propagating through the optical fiber is degraded. At present, since, even a lowest-loss optical fiber has a loss of about 0.2 dB/km, development is in demand for an optical fiber of even smaller optical loss.
On the other hand, there is known a multidimensional periodic structural body having optical propagation characteristics basically affected by frequency or polarization direction, that is, a so-called photonic crystal. J. D. Joannopoulous, et al. disclose the lattice structure of the photonic crystal in xe2x80x9cPhotonic Crystalsxe2x80x9d, Princeton University Press, pp. 122-126, 1995, and disclose a resonant cavity utilizing a photonic band gap in U.S. Pat. No. 5,784,400. Further, Ulrikc Gruning, et al. disclose an optical structure using photonic band gap in W097/04340.
However, there are no literatures which disclose an optical fiber having a structure which does not depend on the refractive index of a core and comprises a cladding having a photonic band gap.
Under such circumstances, an object of the present invention is to provide an optical fiber which is not affected by nonlinear optical phenomena or material dispersion, therefore has a significant effect in transmission of high-speed and high-power light waves.
In the first aspect of the present invention, an optical fiber comprises a core having an area of several times an optical wavelength, and a cladding disposed around the core in which a diffraction grating is arranged at least in a peripheral area adjacent to the core and has a grating period (interval) equal to xc2xd the optical wavelength, namely photonic band gap structure.
In the second aspect of the present invention, an optical fiber comprises a hollow core having an area of several times an optical wavelength, and a cladding disposed around the core in which a diffraction grating is arranged at least in a peripheral area adjacent to the core and has a grating period equal to xc2xd the optical wavelength.
In the third aspect of the present invention, an optical fiber comprises a core having an area of several times an optical wavelength, and a cladding disposed around the core in which a diffraction grating is arranged at least in a peripheral area adjacent to the core and has a grating period equal to xc2xd the optical wavelength, wherein the core and the cladding medium are equal in refractive index, and the diffraction grating in the cladding has a grating structure in which a material of a high refractive index is embedded in a medium of a low refractive index.
In the present invention, the core has an area of about several times the optical wavelength, preferably of 10 to 50 microns. By setting the area of the core to about several times the optical wavelength, the allowable incident optical power of the optical fiber can be increased.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.